Quenching is defined as the process of rapid cooling from the austenitic temperature range at rates so fast that diffusion control phase transformations cannot take place. The resulting microstructure would be desirable to be martensitic. The transformation to martensite starts only when the pipe is cooled below the martensitic start temperature (Ms), and is completed only when the pipe is cooled below the martensitic finishing (Mf) temperature. It is desired that the transformation occur through the wall, meaning that the interior of the pipe is also cooled at a fast enough speed to guarantee transformation.
Austenitic temperature range depends on the steel composition. The selection of this temperature is in general the minimum required to guarantee that the transformation will occur, but not too high as to exert grain growth in the material, resulting in loss of toughness and modification of the cooling rate required for quenching. See Table 1 below taken from EP Patent 2778239 for critical cooling rates for different steel chemistries. They are indicated as CR90, indicating the cooling rate that the device should impose in the material should be greater than the CR90 in order to guarantee more than 90% transformation into martensite. As used in this patent application, “CR90” and CR90M″ are interchangeable terms wherein the CR stands for cooling rate and 90 stands for 90% martensite and M stands for martensite. Therefore, CR90 and/or CR9OM is the cooling rate (generally provided in degrees C. per second) for a given steel composition to guarantee 90% martensite in the tube.
TABLE 1Critical Cooling Rates (CR 90) to have more than 90% martensitefor selected steel compositions.CMnSiCrMoCR90AdequateSteel(wt %)(wt %)(wt %)(wt %)(wt %)Other(*C/s) Hardenability?STD10.130.800.350.520.13Ni, Cu, >100NoTiSTD20.140.800.330.550.10Ni, Cu, >100NoNb—TiSTD30.140.800.340.570.32Ni, Cu,50NoNb—TiCMn10.172.000.20———30YesCMn20.251.600.20———30YesBTi10.171.600.20——B—Ti30YesBTi20.251.300.20——B—Ti25YesCrMo10.171.000.251.000.50—25YesCrMo20.250.600.201.000.50—23YesCrMoBTi10.170.600.201.000.50B—Ti25YesCrMoBTi20.240.400.151.000.25B—Ti25YesCrMoBTi30.240.400.151.000.50B—Ti15YesCrMoBTi40.260.600.160.500.25B—Ti30Yes
Cooling through tubing wall: in some situations, the interior surface of the tubing should be cooled at elevated cooling rates also. EP patent application EP2778239A1 discloses data on the average cooling rate of tubes treated in an industrial quenching heads facility (sprays of water cooling the tube from the external surface). FIG. 4 of the present application (reproduced from FIG. 3 of EP 2778239A1) illustrates cooling rates shown as a function of the pipe Wall Thickness (WT). The shaded area in FIG. 4 corresponds to the wall thickness range typical of coiled tube applications. It is clear that the Cooling Rate in the interior surface decreases as the Wall Thickness increases. When selecting steel chemistries suitable to have more than 90% tempered martensitic, the critical cooling rate of the alloy should be equal or lower than 30° C./s (for this quenching head). If the critical cooling rate of the alloy is equal to 30°/sec, then all gauges typical from continuous tube will typically quench at a higher cooling rate in the interior diameter (ID) (if the heat transfer of the quenching heads is achieved) and quenching is guarantee. The required cooling rate in the ID of the heavier wall product is equal to the critical cooling rate.
A coiled tubing is a continuous metal tube (pipe) typically about 15,000 feet long, but length can be between 5,000 feet to about 40,000 feet. Typically the continuous tube is coiled about a support structure, as known in the art for transportation and further deployment to a well location and then deployment into a wellbore. In certain applications, a heat treatment is applied to the coiled tubing consisting of one or more series of heating and cooling the continuous tube to produce metallurgical changes in the material of the tube that result in the definition of the mechanical properties of the continuous tube. The continuous tube could be heat treated without uncoiling the product, but this method would possess limitations on the capability to achieve uniform properties, as well as the management of tension in the material of the tube that could arise due to change in volume associated to the heating, cooling and phase transformations.
An alternative heat treatment requires the continuous tube to be uncoiled on one end, then heat treated and then coiled at the exit of the heat treating process. When the heat treatment includes a quenching process (the rapid cooling from austenitic temperatures as discussed above) the tube should be subjected to elevated cooling rates that result from the application of a fluid to the heated tube.
In general, continuous tubes (e.g. coiled tubing) are quenched using two methods: (a) quenching heads and/or (b) tanks.
In a prior art quenching head process, eductors (a device for inducing a flow of a fluid from a chamber or vessel by using the pressure of a jet of water, air, steam, etc., to create a partial vacuum in such a way as to entrain the fluid to be removed) are typically placed in distribution lines that are fed by a single pipeline. When one distribution pipe entrance gets clogged due to scale and/or a failure of filtration, a complete set of aligned eductors will stop cooling the tube, and there will be a lower cooling rate of the section of the continuous tube running below such defective educator(s). n such a prior art system, a failed educator of a quenching head system that that would result in inconsistent cooling of the tube, can be overcome by rotating a tube about its longitudinal axis (suitable for pipe of a maximum length of about 42 feet) in the cooling tank. However, rotating a continuous tube of a coiling tubing in a cooling tank is not technical feasible.
In a prior art tank quenching process, the tube is submerged inside a cooling tank. As noted above, a tube of a length of up to about 42 feet may be rotated about its longitudinal axis in order to increase the heat transfer, and alternatively fluid may be jetted inside the tube to help heat extraction from the interior surface.
The cooling heterogeneities of the quench head system may be eliminated by using the tank quenching process. However, in order to accommodate a continuous tube of coiled tubing, a very large tank is needed in order to quench the total length of continuous tube in a coiled tubing. In the case of a coiled tubing which is not uncoiled and entirely immerged into a cooling fluid in a cooling tank, the heat extraction is limited to contacting the outside surface of the coiled tubing.
Therefore, a need exists for an improved quenching tank system for a continuous heated tube of a coiled tubing.